In the early days, screens were often hand-stretched, and if a printer paid any attention at all to tension, he might casually drop a quarter on the screen. If the quarter bounced, mesh tension was considered adequate. (If you're tempted to try this experiment, you'll find a quarter tossed onto the most worn and decrepit screen in your shop will probably still produce a pretty good bounce.)

As screen printing advanced technically, it was quickly seen that higher-tensioned screens produced superior results. This led to the development and manufacture of meshes capable of working under higher and higher tensions, and today, we commonly work with mesh tensioned far above those used in the early days of screen printing.

Mesh tension directly affects so many of the variables in the screenprinting process, it's one of the first thing to check when things go wrong. For example, if the edge definition in your print is awful, or the detail isn't what it should be, one of the first things you should do is check the tension in your screens. If the color varies markedly in solid areas, poor screen tension may be at fault. If your screens are wearing out too quickly, if your stencils develop pinholes, or if you're having a hard time registering colors in multi-color work, check your screen tension.

Tension can have a direct impact on fundamental mesh characteristics, even something as basic as mesh count. For example, manufacturers often specify mesh counts on the basis of untensioned mesh, but when you tension that mesh to spec, the mesh count may drop by as much as 30 threads per inch. Mesh openings, on the other hand, tend to increase in size as tension increases, and those openings have a direct bearing on ink deposit, because every drop of ink printed onto your substrate has to pass through them. Not only how much tension but how it is applied becomes vital in determining the squareness and consistency of those openings. For example, tension has to be equal in both warp (longways) and weft (crossways) directions or mesh openings may not be uniform in shape, a factor that becomes increasingly important as a printer progresses to printing finer lines, halftones, and four-color work.

Almost every mesh has a different ideal tension, which will vary on the basis of thread count and thread diameter. Manufacturers usually publish specifications for every mesh they produce, and it is a fairly narrow range. Professional screen stretchers use a device called a tension meter or newton meter to check tension, and normally, they will take a number of measurements with it to insure that tension remains consistent throughout the screen. Tension is measured in units called newtons, a measurement of resistance, and meter readings record how much the mesh deflects under a known weight, usually the weight of the meter itself. The higher the tension the less the mesh deflects and the higher the meter reading. For what you'd pay for a reliable tension meter you could probably buy a good computer monitor. Which means that tension meters are rarely found outside of a professional screen makerís shop where self-tensioning frames are in use, or a high-volume printing operation.

How properly tensioned screens save you money
Screens with proper tension can do more than simply eliminate puzzling printing problems; they can help make your entire screenprinting operation more efficient.

Some kind of mesh tension has to exist for the screenprinting process to work. There are two possible sources of that tension, either the screen or the squeegee. A printer working with an under-tensioned screen really has no choice except to increase the off-contact distance and push harder with his squeegee to supply that missing tension. When working with a properly tensioned screen, a printer doesn't have to push down as hard, and he can pull prints with less effort, so print jobs run faster and easier.

Tension, however, begins to have an effect on a print job long before the first print is pulled. When coating a properly tensioned screen with direct emulsion, screenprinters find it far easier to apply a smooth and even coat of emulsion. Coatings also tend to be thinner and more consistent across the screen. Because of this, screens will dry more quickly, a great time saver and an especially welcome one when you're trying to build up multiple layers of emulsion on a screen. Finally, when it comes time to expose those screens, that even coating of emulsion will expose more evenly and wash out with less effort.

Because a printer using properly tensioned screens doesn't have to use exaggerated pressure on the squeegee, the mesh takes less of a beating during every print stroke. Consequently, screens last longer. Because screens are subject to less wear during printing, they can deliver more prints per screen and a significant saving to your bottom line.

Some printers who have gotten used to working with under-tensioned screens often worry about higher-tensioned screens being more vulnerable to damage, but that simply is not true. Given reasonable care, they will last at least as long as an under-tensioned screen, and will wear considerably better. Reduced wear is a direct result of the much smaller off-contact distances, which means less wear and tear on the stencil, too. The stencil has to follow the movement of the mesh, but stencil material isn't nearly as flexible. In an under-tensioned screen, the stencil has to flex quite a bit more than it was meant to, causing it to break down earlier and develop tiny cracks. Yes, improper tension can even cause pinholes!

Improved print quality
Proper mesh tension also creates noticeable improvements in print quality, notably images with sharp, well-defined edges and smooth uniform color. This is how it works: As the squeegee passes over the mesh, tension quickly lifts it up off the substrate, getting it up out of the way of the ink. This also allows the ink to flow very slightly, eliminating mesh marks and leaving a smoother ink surface. Prints also tend to be more consistent because the reduced off-contact distance means a printer doesn't have to work as hard and he can exert more control during his print stroke. As a bonus, he may even find he uses slightly less ink to print a job.

How tension affects registration
With under-tensioned mesh, a printer will find his squeegee actually drags the fabric with it during the print stroke. In fact, it stretches the mesh slightly, because the mesh simply doesn't have the tension to resist the pull. This not only increases the likelihood of a blurred print, but the stencil also gets pulled right along with the mesh. The result is that the imprint moves a bit further down the substrate. When printing a two- or three-color job, this can become a real problem, especially if the job happens to be one that demands close registration. The images that should just meet, overlap and the imprints that should touch don't.

Furthermore, there will be inconsistencies from print to print depending on how much squeegee pressure was applied. Remember, the squeegee will actually pull the mesh down with it, so variations in squeegee pressure will show up in imprints that have shifted to varying degrees. But another registration problem results from the exaggerated off-contact distances required by under-tensioned mesh. When mesh has to travel a greater distance to reach the surface of the substrate, it becomes harder to control the position of the imprint.

Properly tensioned mesh not only insures greater print-to-print consistency but also makes it easier to match reprinted jobs to earlier runs. If you have to make a new stencil, and both old and new screens are close to recommended mesh tensions, there's a greater likelihood that the stencils will be identical. If you will be adding the screen to your library, the reduced screen wear will give you more prints before you eventually have to reshoot the screen.

With the importance of proper tension in mind, should a printer stretch his own screens? Virtually every newcomer to screen printing imagines he can save lots of money by making his own screens. This is probably one of the biggest time-wasters in the business. Without a professional tensioning system and the guidance of a tension meter there is no way to get screens close to specified tensions. For a beginner, an investment in professionally made screens is a very wise choice indeed, because it eliminates so many of the tension-related variables in the printing process. That leaves him free to concentrate on learning how to control all the other factors in screenprinting. Stretching screens in-house probably only makes sense in a very high-volume shop with an owner prepared to make a considerable investment in equipment and training. Mesh tension is one thing it doesn't pay to take lightly.